Method for producing aluminosilicate glass

ABSTRACT

The invention relates to a method for producing aluminosilicate glass that is devoid of alkali and that has a B 2 O 3  content of between 0 and &lt;5 wt.-% and a BaO content in excess of 5.5 wt.-%. Said method is characterised by the addition of between 0.05 wt.-% and 1.0 wt.-% SnO 2  during the preparation of the mixture.

The invention relates to a process for producing aluminosilicate glasseswith addition of a fining agent to the batch formulation. The inventionalso relates to the glasses comprising the fining agent and to uses ofthe glasses.

Processes for producing glasses consist of the steps of batchformulation, also called batch charging, melting of the glass, andsubsequent hot forming thereof. The term melting also embraces the stepsof fining, homogenizing and conditioning for further processing, whichfollow the melting-in operation.

As applied to melts, fining refers to the removal of gas bubbles fromthe melt. In order to achieve a very high level of freedom fromextraneous gases and bubbles, it is necessary for the melted batch to bethoroughly mixed and degassed. The characteristics of gases and bubblesin the glass melt, and their removal, are described, for example, in‘Glastechnische Fabrikationsfehler’ [glass-making defects], edited by H.Jebsen-Marwedel and R. Brückner, 3rd Edition, 1980, Springer-Verlag,page 195 ff.

There are two fundamentally different fining processes which are commonknowledge: they differ essentially in the way the fining gas isproduced:

In the case of mechanical fining, gases such as water vapour, oxygen,nitrogen or air, are injected through openings in the bottom of themelting unit. This process is known as bubbling.

The most frequent fining processes are the chemical processes.

Their principle, consists in adding to the melt or even to the batch

-   -   a) Compounds which give off gases at relatively high        temperatures in an equilibrium reaction, or    -   b) Compounds which are volatile at relatively high temperatures,        or    -   c) Compounds which decompose in the melt and in doing so release        gases.

As a result, the volume of existing bubbles is increased and theirdistension is intensified.

The first-mentioned group of compounds embraces those known as redoxfining agents, such as antimony oxide and arsenic oxide, for example. Inthe case of this process, which is the most frequently used in practice,the redox fining agents employed comprise polyvalent ions which are ableto exist in at least two oxidation states and which are in atemperature-dependant equilibrium of one another; at high temperatures agas, usually oxygen is released.

The second group, made up of compounds which are volatile at hightemperatures owing to their vapour pressure and so exert their effect,includes, for example, sodium chloride and various fluorides. They arereferred to collectively as evaporation fining agents.

The last-mentioned type of chemical fining, i.e. fining by means ofcompounds which decompose and in doing so release gases, referred tohere as decomposition fining agents, includes sulphate fining. Thisfining is known for low-melting glasses, such as for soda-lime glasses,since the commonly used Na₂SO₄ (in the case of mass-produced glasses,also in the form of Glauber's salts, Na₂SO₄-10 H₂O) reacts with the SiO₂that is always present at temperatures which, in comparison with theNa₂SO₄ which is relatively stable on its own, are low, in accordancewith the equationNa₂SO₄+SiO₂→Na₂O·SiO₂+SO₂+½O₂orNa₂SO₄+Na₂S+SiO₂→2Na₂O·SiO₂+SO₂+S.

On grounds not least of the environment, the aforementioned redox finingagent Sb₂O₃ and As₂O₃ are not readily used.

Moreover, they are unsuitable for fining glasses that are to be used asbulb glasses for halogen lamps on account of the fact that they arereadily reducible and, in the course of hot processing in the flame thecrimp, i.e. the melt formed between glass and current supply leads,acquires a brown discolouration owing to the reduction of the antimonyoxide or arsenic oxide. Moreover, Sb₂O₃ in particular, at least inrelatively high fractions, promotes unwanted blackening on the inside ofthe bulb, which originates from tungsten deposition owing to disruptionsin the halogen cycle.

As₂O₃ and Sb₂O₃ are also unsuitable for the fining of flat glassesproduced on a float unit, since under the reducing conditions whichprevail in such a process they would be reduced to elemental As or Sb,respectively, on the float bath.

An alternative redox fining agent is CeO₂. However, it is relativelyexpensive and, especially in relatively large amounts, may lead tounwanted fluorescence phenomena in the glass and to yellowing of theglass.

Redox fining is tied to the temperatures at which the correspondingredox processes proceed, owing to the thermodynamic circumstances. Formany glass melts, such as the melts of soda-lime glasses and otherrelatively low-melting glasses (e.g. borate glasses, lead glasses) thesefacilities are sufficient.

However, in the case of glasses with melting temperatures (temperatureat a viscosity of about 10² dPas) of between about 1550° C. and 1650°C., which for adequate fining means fining temperatures of more than1600° C., the bubbles form more poorly owing to increased glass meltviscosity. The growth tendency of such bubbles is lower, and they risemore poorly than at lower viscosities. Accordingly, fine bubbles areformed which are very difficult if not impossible to remove even byreducing the throughput or by means of higher temperatures, so makingsuch glasses unusable. The reason for this is that the absorption effectof some chemical redox fining agents, e.g. Sb₂O₃, i.e. the ability toreabsorb the oxygen or other gases from the fine bubbles on cooling andthus to remove those gases, is inadequate for many high-melting glasses.

The possibilities of increasing the temperatures for the purpose ofreducing viscosity and of prolonging the melting and fining times, whichexist in principle to a certain extent, are uneconomic, since thelast-mentioned measure, for example, would excessively reduce the meltoutputs.

The abovementioned high-melting glasses include in particularaluminosilicate glasses, i.e. silicate glasses containing at least 10%by weight Al₂O₃, especially alkali-free aluminosilicate glasses, inparticular aluminosilicate glasses containing little or no B₂O₃,especially those having a relatively high BaO content, particularlyaluminosilicate glasses which, owing to the high temperature stabilityassociated with the high melting temperatures, are used as substrateglasses in, for example, display technology, or in particular as lampglasses, for halogen lamps for example.

A further redox fining agent is SnO₂, which forms fining gas inaccordance with the reaction equation SnO₂→SnO+½O₂. Gases such as CO₂which are dissolved in the melt diffuse into an O₂ bubble formed in thisway. Those bubbles which are large enough rise by distension to theglass surface, where the gas is emitted from the melt. Even after thefining process, small bubbles remain in the melt. If the temperature islowered, the tin oxide with higher valency is formed again and takes upoxygen from the bubbles still present in accordance with equationSnO+½O₂→SnO₂. This, in other words, is a reabsorption.

SnO₂ is a good nucleating agent and promotes crystallization, so thatwhen SnO₂ is used as a fining agent the likelihood of crystal-inducedglass defects and of elimination of cassiterite phases is increased.

The patent literature has already disclosed aluminium-containing glasseswhich, in some cases along with other fining agents include SnO₂.

For instance, JP 10-130034 A describes aluminoborosilicate glasses whichbesides SnO₂ mandatorily comprise As₂O₂, while JP 10-114538 A describesaluminoborosilicate glasses which besides SnO₂ mandatorily compriseSb₂O₃.

JP 11-43350 A describes aluminoborosilicate glasses which in addition toSnO₂ mandatorily contain Sb₂O₃ and Cl₂. JP 10-324526 A describesaluminoborosilicate glasses to which a component from the groupconsisting of Fe₂O₃, Sb₂O₃, SnO₂ and SO₃ and one from the groupconsisting of Cl and F are added and which still include an As₂O₃fraction.

JP 10-139 467 A describes aluminoborosilicate glasses containing from0.1 to 20 mol % of SnO₂ and/or TiO₂ and/or ZrO₂.

JP 10-59741 A describes SnO₂-containing aluminoborosilicate glasseswhich, like the glasses of the other cited documents, may vary within arelatively wide range in terms of their composition.

Aluminoborosilicate glasses containing SnO₂ are also already known fromthe applicant's publications DE 196 03 698 C1, DE 196 17 344 C1, DE 19601 922 A1 and DE 197 39 912 C1.

A common feature of these glasses is that they contain high levels ofB₂O₃, thereby lowering the melting temperature.

As a result it is possible to prevent the Sn²⁺ which is formed in thecourse of fining from being reduced further to the metal, since hightemperatures would more strongly stabilise the low oxidation states ofpolyvalent ions. Elemental Sn would lead to the formation of alloy atthe electrodes of the melt end.

It is an object of the invention, then, to provide a process forproducing aluminosilicate glasses where the glass melt is effectivelyfined, i.e. which results in glass of high quality in terms of absenceor paucity of bubbles and permits fining of the glass melts, especiallythose of glasses which melt at high temperatures.

This object is achieved by the process according to claim 1.

In the process for producing an alkali-free aluminosilicate glass,comprising the steps of batch formulation, melting of the glass andsubsequent hot forming, the term melting embracing not only the meltingof the raw materials and cullet but also the subsequent steps of finingand homogenizing, at least one fining agent, and specifically between0.05% by weight and 1.0% by weight of SnO₂, is added to the batch.

Preference is given to an addition of from 0.1 to 0.5% by weight ofSnO₂.

The tin oxide here is used in the form of tetravalent tin dioxide SnO₂,which is held in this oxidation state by additions of nitrate to thebatch. At the higher temperatures in the fining section of the melt end,the tin ions undergo partial transition to the divalent state, with theoxygen bubbles formed rising and so contributing to fining by virtue ofthe fact that gases dissolved in the melt diffuse into these bubbles andso are removed from the glass. Very small bubbles which have not risenare reabsorbed at the end of the fining phase, known as the takedownphase, i.e. at low temperatures, by the tin monoxide, SnO, that ispresent at that point and which is oxidized to SnO₂ in the course ofthis reabsorption.

The nitrate for stabilizing the tetravalent tin ions may be introducedby way of various glass components: e.g., in the form of Ba(NO₃)₂,Mg(NO₃)₂, Ca(NO₃)₂, Al(NO₃)₃ or the like.

The process of the invention is used for producing aluminosilicateglasses—by which are understood silicate glasses containing at least 10%by weight Al₂O₃—which contain between 0 and <5% by weight B₂O₃ and from5.5% by weight BaO.

The process of the invention is used to produce glasses which, exceptfor customary impurities, are free from alkali metal oxides. Theimpurities fraction may be minimised by using low-alkali raw materialsand also by clean conditions at the batch formulation stage and in thebatch-feeding section of the melt end. Accordingly, the term alkali-freeshould be understood here to refer to an alkali metal oxide content ofnot more than 0.1% by weight.

The process is particularly suited to the production of aluminosilicateglasses having melt temperatures >1650° C.

The process is used in particular to produce aluminosilicate glasseshaving thermal expansion coefficients after α_(20/300)<7.5·10⁻⁶/K,generally glasses having high Al₂O₃ contents, preferably ≧12% by weightAl₂O₃, with particular preference ≧13.5% by weight Al₂O₃, which leads toan increase in the melting temperature and fining temperature.

The process is used in particular to produce what are known as hardglasses, i.e. glasses having high transition temperatures Tg (>600° C.)and low thermal expansion (α_(20/300)<5.5·10⁻⁶/K).

The process of the invention is preferably used to produce glasseshaving a composition in the following range:

-   SiO₂ 46–64, Al₂O₃ 12–26, B₂O₃ 0–<5, MgO 0–7, CaO 3–14, SrO 0–11, BaO    6–25, ZrO₂ 0–5, TiO₂ 0–0.6, P₂O₅ 0–9, SnO₂ 0.05–1.

The process of the invention is therefore used in particular to produceglasses which are suitable both as substrate glasses for displaytechnology and for photovoltaics and as lamp bulb glasses for halogenlamps.

The person skilled in the art knows how to conduct the process step ofbatch formulation with appropriate raw materials in such a way as toobtain a glass having the specified composition. For instance, as isknown, P₂O₅ has a high volatility, so that up to 20% can evaporate whenthe glass is melted, which the skilled person will take into account atthe batch formulation stage.

The process is used in particular to produce glasses with a compositionin the following range (in % by weight based on oxide):

-   SiO₂ 46–63; Al₂O₃ 12–25, preferably >17; MgO 0–5; preferably 0–4;    CaO 3–14, SrO 0–11; BaO 6–15, where MgO+CaO+SrO+BaO ≦25, where    SrO+BaO ≧10; ZrO₂ 0.1–5; P₂O₅ 0.1–9, preferably 0.5–9, SnO₂ 0.05–1.

For the abovementioned uses, especially as lamp bulb glass, particularlysuitable glasses are those which are produced by the process of theinvention and have compositions from the following range (in % by weightbased on oxide):

-   SiO₂>55–64, Al₂O₃ 13.5-15.5, B₂O₃ 0–<5, MgO 0–7, CaO 5–14, SrO 0–8,    BaO 6–17, ZrO₂ 0–2, TiO₂ 0–0.5, SnO₂ 0.05–1.

In this context, glasses with a composition from the following range (in% by weight based on oxide):

-   SiO₂ 59–62, Al₂O₃ 13.5–15.5, B₂O₃ 3–<5, MgO 2.5–5, CaO 8.2–10.5, BaO    8.5–9.5, ZrO₂ 0–1.5, TiO₂ 0–0.5, SnO₂ 0.05–1.    are particularly suitable as bulb glasses for halogen lamps with    bulb temperatures of not more than 660° C., while glasses with a    composition in the following range (in % by weight based on oxide):-   SiO₂>58–62; Al₂O₃ 14–17.5, preferably 15–17.5; B₂O₃ 0–1, preferably    0.2–0.7; MgO 0–7, preferably 0–<1; CaO 5.5–14; SrO 0–8; BaO 6–17,    preferably 6–10; ZrO₂ 0–1.5, preferably 0.05–1; TiO₂ 0–0.5, SnO₂    0.05–1.    are suitable for halogen lamps with bulb temperatures of more than    660° C.

The glasses produced by the process of the invention may furthercomprise the following polyvalent compounds: up to 2% by weight of MoO₃,up to 2% by weight of WO₃, up to 0.6% by weight of CeO₂, up to 0.2% byweight MnO₂, up to 0.5% by weight of Fe₂O₃, and up to 0.2% by weight ofV₂O₅. The sum of these components should be between 0 and 3% by weight.

As already elucidated for SnO₂, the compounds in the glass may bepresent in different oxidation states; as for SnO₂ as well, however,their content is in each case stated for the specified oxidation state.

It is a particular advantage that in the process of the invention noevaporation fining agents such as chlorides and fluorides are used.Owing to the high solubility in glass of the fluorides in particular,the amounts needed for effective fining would be so large that thephysical and chemical properties of the glasses would be modified tosuch an extent as to excessively lower their thermal stability andviscosity. If borosilicate glasses containing Cl⁻ were reheated, such asin cases of remelting, it would be possible for white coatings, known aslamp rings, to occur.

It is a particular advantage that there is no need for decompositionfining agents in the process of the invention.

It is a particular advantage that with the process of the invention itis possible to produce glasses which except for unavoidable impuritiesare free from readily reducible constituents, especially As₂O₃, Sb₂O₃,CdO, PbO, Bi₂O₃. The avoidance of these components is not onlyadvantageous on grounds of environmental protection but also permits hotforming on a float unit in the process of the invention, for example forthe production of substrates for display technology or photovoltaics.

The process of the invention is particularly advantageous for theproduction of alkali-free halogen lamp glasses which, because of thehigh temperature stability they are required to have, have high meltingtemperatures. Here, the process is able to be a complete substitute forSb₂O₃ fining.

In halogen lamp glasses produced by the process, even at the high lampoperating temperatures mentioned and following prolonged usage of thelamp, there is a reduction of the blackening of the inside of the bulbwhich occurs as a result of deposition of tungsten as a consequence ofdisruptions in the halogen cycle. In other words, the regenerativehalogen cycle in the lamp can be maintained for longer than is the casewith Sb₂O₃-fined glasses. Also, there is no browning of the crimp in thecourse of hot processing in the flame.

The abovementioned process step of hot forming includes not only thefloating and tube drawing but also a very wide variety of customary hotforming methods such as drawing, into tubes or strips, or floating orrolling, casting, blowing, pressing, as appropriate to the intended useof the glasses, flat glasses or hollow glasses produced. Here again, theperson skilled in the art is readily able to select the appropriateglass composition and to choose accordingly the parameters of the hotforming process step.

The step in the production process of the invention that is essential tothe invention, namely the addition of the stated amount of SnO₂, resultsin very effective fining, which is reflected in the outstandingquality—i.e. paucity of bubbles—of the glasses produced.

Quite unexpectedly and in contrast to all experience to date in thefield of the production of high-melting glasses, the process of theinvention is outstandingly suitable for the production of high-meltingalkali-free low-boron or boron-free aluminosilicate glasses.

Surprisingly, in accordance with the process of the invention, theseglasses may be produced with melting temperatures >1650° C. withoutreduction of the tin ions to elemental tin.

The glasses produced are free from crystallization defects. Glass of anoutstanding quality which meets the specifications for lamp bulb glassesis obtained.

There is no formation of alloy at the Pt electrodes and there is no Ptelectrode corrosion.

Accordingly, the process of the invention comprises efficient andcost-effective fining of the glasses. Glass melts in particular which atthe customary fining temperatures have a high viscosity, namely melts ofalkali-free, high BaO content, boron-free or low-boron glasses, andwhich are therefore difficult to fine, may be fined to glasses of highquality with high melt outputs.

The invention is to be elucidated further with reference to workingexamples and comparative examples.

For all of the examples the following procedure was used:

Using a batch charger, the batch was charged continuously to a melt end,the amount charged being regulated by the level of the liquid glass inthe melt end. In the description of the invention, this chargingoperation is included in the term batch formulation. Melting, fining andtake down of the melted glass were carried out in the customary mannerby lowering the temperature. In a working end and a downstream feederchannel—where a spinner is also possible—the glass was conditionedchemically and thermally by stirring.

For the comparative example, a glass whose basic composition was asfollows (in % by weight based on oxide): 59.1 SiO₂; 4.6 B₂O₃; 14.5Al₂O₃; 8.8 BaO; 10.3 CaO; 2.5 MgO; 0.18 Sb₂O₃ was melted at >1630° C.and fined. Raw materials used were oxide and carbonates. 1.5% by weightof the BaO were used in the form of barium nitrate.

The bubble count of the glass thus produced is ≧20/kg of glass andcannot be lowered even by reducing the melt output by 20%. Small andvery small bubbles in particular, referred to as seeds, i.e. the bubbleswhich on the tube have an extended length <1 cm, are the most frequentglass defects in the product.

For Working Example 1, a glass of the same basic composition as thecomparative example but without the Sb₂O₃ and with 0.2% by weight ofSnO₂ was produced. Otherwise, the raw materials used were the same.

The bubble count was reduced to less than 10 per kg of glass, therebyemphasising the improvement of fining by means of SnO₂. The fact thatbesides a few relatively large bubbles there were virtually no fineseeds now, and that the desired effect—namely the reduction in seedinessin favour of a few large bubbles, which were able to rise more easilyand which have left the melt—has occurred are a further sign of the verygood fining effect.

As a further example (Working Example 2) a glass of the following basiccomposition (in % by weight based on oxide): 60.7 SiO₂; 0.3 B₂O₃; 16.5Al₂O₃; 7.85 BaO; 13.5 CaO; 1.0 ZrO₂ was produced. 0.25% by weight ofSnO₂ was added. The melting temperature was >1640° C.; otherwise, theproduction conditions were the same as those mentioned above. Hereagain, under comparable melting conditions, glass with <10 bubbles/kg ofglass was obtained.

1. A process for producing an alkali-free aluminosilicate glasscomprising charging an alkali-free aluminosilicate glass batchcomprising, in % by weight based on oxide,B₂O₃<5,BaO>5.5, andSnO₂, 0.05–1.0; and subsequently melting and hot forming a resultantalkali-free aluminosilicate glass.
 2. A process according to claim 1,wherein the alkali-free aluminosilicate glass batch (in % by weightbased on oxide) comprises: SiO₂ 46+14 64 Al₂O₃ 12+14 26 B₂O₃ +12 +120+14 +21 5 MgO 0+14 7 CaO +11 3+14 14 SrO +11 0+14 11 BaO +11 6+14 25ZrO₂ 0+14 5 TiO₂ +11 +12 0+14 0.6 P₂O₅ 0+14 9 SnO₂ 0.05+14 1.+10


3. A process according to claim 2, wherein the alkali-freealuminosilicate glass batch (in % by weight based on oxide) comprises:SiO₂ +22 55+14 64+12 +12 Al₂O₃ 13+14 18 B₂O₃ +12 +12 0+14 +21 5 MgO 0+147 CaO +11 5+14 14 SrO 0+14 8 BaO +11 6+14 17 ZrO₂ 0+14 2 TiO₂ +11 +120+14 0.5 SnO₂ 0.05+14 1.+10


4. process according to claim 3, wherein the alkali-free aluminosilicateglass batch (in % by weight based on oxide) comprises: SiO₂ 59+14 62Al₂O₃ 13.5+14 15.5 B₂O₃ +12 +12 3+14 +21 5 MgO 2.5+14 5+11 +12 CaO +118.2+14 10.5 BaO 8.5+14 9.5 ZrO₂ +11 +12 0+14 1.5 TiO₂ +11 +12 0+14 0.5SnO₂ 0.05+14 1.+10


5. A process according to claim 3, wherein the alkali-freealuminosilicate glass batch (in % by weight based on oxide) comprises:SiO₂ +22 58+14 62+12 +12 Al₂O₃ +11 +12 14+14 17.5 B₂O₃ 0+14 1 MgO 0+14 7CaO 5.5+14 14+12 SrO 0+14 8 BaO +11 6+14 17 ZrO₂ +11 +12 0+14 1.5 TiO₂+11 +12 0+14 0.5 SnO₂ 0.05+14 1.+10


6. A process according to claim 5, wherein the alkali-freealuminosilicate glass batch (in % by weight based on oxide) comprises:SiO₂ +22 58+14 62+12 +12 Al₂O₃ +11 +12 15+14 17.5 B₂O₃ 0.2+14 0.7 MgO+12 +12 0+14 +21 1 CaO 5.5+14 14+12 SrO 0+14 8 BaO +11 6+14 10 ZrO₂0.05+14 1+12 +10 TiO₂ +11 +12 0+14 0.5 SnO₂ 0.05+14 1.+10


7. A process according to claim 2, wherein the alkali-freealuminosilicate glass batch (in % by weight based on oxide) comprises:SiO₂ 46+14 63 Al₂O₃ 12+14 25 MgO 0+14 5 CaO +11 3+14 14 SrO +11 0+14 11BaO +11 6+14 15 with MgO +30 +0 CaO +30 +0 SrO +30 +0 BaO +23 25 withSrO +30 +0 BaO +24 10 ZrO₂ 0.1+14 5+11 +12 P₂O₅ 0.1+14 9+11 +12 SnO₂0.05+14 1.+10


8. A process according to claim 1, wherein the alkali-freealuminosilicate batch comprises 0.1%–0.5% by weight of SnO₂.
 9. Aprocess according to claim 1, wherein no evaporation fining agents areadded.
 10. A process according to claim 1, wherein no decompositionfining agents are added.
 11. A process according to claim 1, wherein thealkali-free aluminosilicate glass batch (in % by weight based on oxide)further comprises: CeO₂ +11 +12 0+14 0.6 MoO₃ 0+14 2 WO₃ 0+14 2 V₂O₅ +11+12 0+14 0.2 MnO₂ +11 +12 0+14 0.2 Fe₂O₃ +11 +12 0+14 0.5 where CeO₂ +30+0 MoO₃ +30 +0 WO₃ +30 +0 V₂O₅ +30 +0 MnO₂ +30 +0 Fe₂O₃ +12 0+14
 3.


12. A process according to claim 1, wherein the alkali-freealuminosilicate glass batch is free from As₂O_(3,) Sb₂O_(3,) CdO, PbO,Bi₂O₃, except for unavoidable impurities.
 13. Alkali-freealuminosilicate glass having a composition as follows (in % by weightbased on oxide) SiO₂ 46+14 64 Al₂O₃ 12+14 26 B₂O₃ +12 +12 0+14 +21 5 MgO0+14 7 CaO +11 3+14 14 SrO +11 0+14 11 BaO +11 6+14 25 ZrO₂ 0+14 5 TiO₂+11 +12 0+14 0.6 P₂O₅ 0+14 9 SnO₂ 0.05+14 1.+10


14. Alkali-free aluminosilicate glass according to claim 13, having acomposition as follows (in % by weight based on oxide): SiO₂ +22 55+1464+12 +12 Al₂O₃ 13+14 18 B₂O₃ +12 +12 0+14 +21 5 MgO 0+14 7 CaO +11 5+1414 SrO 0+14 8 BaO +11 6+14 17 ZrO₂ 0+14 2 TiO₂ +12 +11 0+14 0.5 SnO₂0.05+14 1.+10


15. Alkali-free aluminosilicate glass according to claim 14, having acomposition as follows (in % by weight based on oxide): SiO₂ 59+14 62Al₂O₃ 13.5+14 15.5 B₂O₃ +12 +12 3+14 +21 5 MgO 2.5+14 5+12 +11 CaO +118.2+14 10.5 BaO 8.5+14 9.5 ZrO₂ +11 +12 0+14 1.5 TiO₂ +11 +12 0+14 0.5SnO₂ 0.05+14 1.+10


16. Alkali-free aluminosilicate glass according to claim 14, having acomposition as follows (in % by weight based on oxide): SiO₂ +22 58+1462+12 +12 Al₂O₃ +11 +12 14+14 17.5 B₂O₃ 0+14 1 MgO 0+14 7 CaO 5.5+1414+12 SrO 0+14 8 BaO +11 6+14 17 ZrO₂ +11 +12 0+14 1.5 TiO₂ +12 +11 0+140.5 SnO₂ 0.05+14 1.+10


17. Alkali-free aluminosilicate glass according to claim 16, having acomposition as follows (in % by weight based on oxide): SiO₂ +22 58+1462+12 +12 Al₂O₃ +11 +12 15+14 17.5 B₂O₃ 0.2+14 0.7 MgO +12 +12 0+14 +211 CaO 5.5+14 14+12 SrO 0+14 8 BaO +11 6+14 10 ZrO₂ 0.05+14 1+10 +12 TiO₂+12 +11 0+14 0.5 SnO₂ 0.05+14 1.+10


18. Alkali-free aluminosilicate glass according to claim 13, having acomposition as follows (in % by weight based on oxide): SiO₂ 46+14 63Al₂O₃ 12+14 25 MgO 0+14 5 CaO +11 3+14 14 SrO +11 0+14 11 BaO +11 6+1415 where MgO +30 +0 CaO +30 +0 SrO +30 +0 BaO +23 25 where SrO +30 +0 +0BaO +24 10 ZrO₂ 0.1+14 5+11 +12 P₂O₅ 0.1+14 9+11 +12 SnO₂ 0.05+14 1.+10


19. Alkali-free aluminosilicate glass according to claim 13, furthercomprising CeO₂ +11 +12 0+14 0.6 MoO₃ 0+14 2 WO₃ 0+14 2 V₂O₅ +11 +120+14 0.2 MnO₂ +11 +12 0+14 0.2 Fe₂O₃ +11 +12 0+14 0.5 where CeO₂+0 +30+0 MoO₃+0 +30 +0 WO₃+0 +30 +0 V₂O₅+0 +30 +0 MnO₂+0 +30 +0 Fe₂O₃ +12 0+143.


20. Alkali-free aluminosilicate glass according to claim 13, which,except for unavoidable impurities, is free from As₂O₃, Sb₂O₃, CdO, PbO,Bi₂O₃.
 21. A process for producing an alkali-fre aluminosilicate glass,comprising melting a composition comprising (in % by weight based onoxide): B₂O₃ +21 5; BaO +22 5.5; and SnO₂ 0.05+14 1.0

and glass components constituting the aluminosilicate glass.
 22. Aprocess according to claim 21, further comprising batch charging thecomposition wherein the SnO₂ is added during batch charging.
 23. Aprocess according to claim 21, wherein the SnO₂ is added during melting.24. A process according to claim 22, further comprising hot forming aglass made after melting.
 25. A process according to claim 21, whereinthe composition comprises 0.1%–0.5%, by weight based on oxide of SnO₂.